Today's electricity grid relies on a number of different sources of power generation. Traditional sources include coal fired turbines, gas fired turbines, oil fired turbines, hydro dams, nuclear reactors and the like. However as shown in FIG. 1, if energy demand increases by 5% each year fossil fuel reserves 100 will run out by approximately 2050. As a result alternative energy sources and more environmentally friendly generation techniques are coming into favour, including wind farms, solar farms, tidal generators, etc. However both traditional and green generation suffer from the same problem, that they need to be located remotely a long way from where the demand is. Moreover, they suffer from intermittency.
For traditional generation the remote location is required either due to pollution concerns or the large amount of space required. For green generation often the natural resource is in the remote location, or it is due to the large amount of space required. In either case the distance the electricity needs to be transmitted causes difficulty in terms of the cost of the infrastructure and the electrical losses that may result. Also more reliance on fewer large scale generation sources may lead to lower reliability for the system and less stability.
One option is smaller generation plants closer to the demand centres. However it is difficult to situate even small generation plants close to population centres without raising noise, pollution or risk concerns with the inhabitants.
At the other end of the scale it is has also been proposed to integrate green energy generation into buildings and in some cases to design the building to have zero energy consumption overall. FIG. 2 shows a comparison 200 between the typical solar Photovoltaic (PV) generation profile 202 over 24 hours and the building load profile 204. This shows a temporal mismatch where sometimes the grid is required to supply 206 or absorb 208 energy at certain times of the day.
When generation does not match demand, the frequency of the grid tends to vary. This needs regulation. Regulating reserves are put in use to overcome the problems. Such reserves are fast responding power plants on stand by. However, the response time required when renewable are used in generating mix is very small. Due to moving cloud, the solar PV output can drop very quickly and change to a new value causing a fast large increase in grid load. Similarly during peak sunshine hours there will be an large excess of supply from the PV array (i.e. the building appears as a generator not a load). If either were to happen in an aggregated form, a fast acting regulating reserve would be needed to keep the grid frequency within mandatory limits. Thus if building integrated PV arrays become highly popular this may cause challenges for the grid.
Another complication is that the metering and billing system might have to be able to cope with two way power flow. Also power systems need to be rated off the peak power requirements, even with building integrated generation, the remote generation and grid capacity might need to be rated for if there was no building integrated generation. As an example if the whole city was covered by cloud (and the building integrated generation was all solar) the shortfall would have to come from traditional generation. Thus the system might need generation capacity (including that of the building integrated generation) twice that of the peak demand, for redundancy (this could be somewhat reduced by diversity into wind, micro hydro, geothermal etc).
Thus it would be desirable for such green generation systems, particularly those building integrated, to incorporate some mechanism to avoid the demand/supply mismatches being propagated into the grid.
Several attempts have been made to avoid the problems mentioned. The simplest is a battery bank. As can be seen in FIG. 3 the Ragone plot 300 shows batteries 302 have high energy density 304 but suffer from the disadvantage that their power density 306 is not very high. High energy density 304 means that the steady state capacity for energy storage is relative high. Low power density 306 means that high frequency variations such as changes in demand that occur over milli seconds cannot be supplied or absorbed by battery bank 302. Thus with a battery bank 302 high frequency variations will still be propagated to the grid.
Similarly other energy storage mediums suffer from problems associated with either energy density 304 or power density 306. Capacitors 308, fuel cells 310 and electrolytic capacitors 312 are all shown in disparate locations in the plot 300 which do not intersect.
Also since gaps 314 exist in the Ragone plot 300, some scenarios of load profile and PV profile may not be able to be accommodated with current technology.
Prior art attempts to solve this include US patent publication numbers 2007/0062744 and 2010/0133025 which propose a composite energy storage system which combines a battery bank and a capacitor bank. However they suffer from the disadvantage that it is a rigid configuration that may not be efficient or optimal in certain circumstances such as the power and energy requirements of a smart micro-grid.
This problem may be exaggerated in an island or an isolated system not connected to the grid. In that case all load demands must always be provided by the green generation system and/or an energy storage system, both high frequency variation in the load and/or generation and steady state.